The present invention relates to an electrophotographic apparatus and a process cartridge, more particularly an electrophotographic apparatus and a process cartridge using a charging scheme wherein an electrophotographic photosensitive member is charged predominantly according to a charging mechanism whereby charges are directly injected into the photosensitive member surface from a charging member contacting the photosensitive member.
In an electrophotographic process, an electrophotographic photosensitive member comprising a photoconductor, such as selenium, cadmium sulfide, zinc oxide, amorphous silicon or an organic photoconductor is subjected to basic or unit processes, such as charging, exposure, development transfer and fixation, and in the charging process, a corona discharge phenomenon caused by applying a high voltage (on the order of DC 5-8 kV) to a metal wire has been conventionally used. According to the corona discharge scheme, however, corona discharge products, such as ozone and NOX, denature the photosensitive member to result in blurring or deterioration of images, or soil the wire to adversely affect the image quality, thus resulting in white dropout or black streaks in images.
Particularly, in the case of an electrophotographic photosensitive member having a photosensitive layer principally comprising an organic photoconductor, which has a lower chemical stability than other photosensitive members, such as a selenium photosensitive member and an amorphous silicon photosensitive member, the organic photosensitive member and amorphous silicon photosensitive member, the organic photosensitive member is liable deteriorate due to chemical reactions, principally oxidation, when exposed to such corona discharge products. Accordingly, when used repetitively in the corona discharge charging scheme, the organic photosensitive member is liable to show a lower printing or copying life, due to the deterioration thereof leading to difficulties, such as image blurring, a lowering in sensitivity and a lower image density due to an increase in residual potential.
Further, the corona discharge charging scheme exhibits a lower charging efficiency as only 5-30% of electricity is utilized as a current flowing toward the photosensitive member and a major portion thereof is directed to a shield plate. For alleviating these problems, contact charging methods not utilizing a corona discharger have been studied, as proposed in JP-A 57-178267, JP-A 56-104351, JP-A 58-40566, JP-A 58-139156, JP-A 58-150975, etc. More specifically, in such a contact charging scheme, a charging member, such as an electroconductive elastic roller, supplied with DC voltage of ca. 1-2 kV from an external supply is caused to contact an electrophotographic photosensitive member, thereby charging the photosensitive surface to a prescribed potential.
The contact charging scheme is disadvantageous compared with the corona charging scheme, in respects of the non-uniformity of charge and the occurrence of dielectric breakdown of the photosensitive member, which result in, e.g., a charging irregularity in a streak shape of ca. 2-200 mm in length and ca. 0.25 mm or below in a direction perpendicular to the moving direction of the photosensitive member, leading to an image defect of a white streak (in a solid black or halftone image) in the normal development scheme or a black streak in the reversal development scheme.
For providing an improved charging uniformity to solve the above-mentioned problem, a method of superposing an AC voltage on a DC voltage and applying the superposed voltage to a charging member has been proposed (JP-A 63-149668). According to the charging method, an AC voltage (Vac) is superposed on a DC voltage (Vdc) to form a pulsating voltage for application, thereby effecting uniform charging.
By ensuring a charging uniformity to obviate image defects, such as white spots in the normal development scheme, or black spots or fog in the reversal developing scheme, according to the superposed voltage charging scheme, the superposed AC voltage is required to have a peak-to-peak potential difference (Vpp) of at least twice a discharge initiation voltage (Vth) according to the Paschen""s law.
However, as the superposed AC voltage is increased in order to obviate the image defects, the maximum applied voltage of the pulsating voltage is increased, and a dielectric breakdown due to discharge is liable to occur even at a slight defect in the photosensitive member. Particularly, in the case of a photosensitive member comprising an organic photoconductor having a lower dielectric strength, the dielectric breakdown is liable to be caused. Similarly as in the DC charging scheme, if such a dielectric breakdown is caused, a white image dropout is caused in the normal development scheme and a black streak image defect is caused in the reversal development scheme, in a longitudinal contact direction (i.e., a lateral direction of a recording material).
Further, also in the DC-AC superposed contact charging scheme, the charging mechanism still relies on a discharge phenomenon across a minute gap, and discharge products, such as NOX or ozone, deteriorate the photosensitive member surface and result in attachment of low-resistivity materials onto the surface, leading to problems, such as image blurring. Further, as the charging member contacts the photosensitive member and the photosensitive member is exposed to a much higher electric field intensity than in the corona charging scheme, a surface layer of the photosensitive member is liable to peel off to result in a shorter life of the photosensitive member.
In order to solve the above-mentioned problems, there has been proposed a charging process wherein charges are directly injected into a photosensitive member without being substantially accompanied by a discharge phenomenon.
The charging scheme wherein direct charge injection to a photosensitive member (which may also be called xe2x80x9cinjection chargingxe2x80x9d) is predominant is substantially different from the above-mentioned charging scheme wherein the discharge is predominant (which may also be called xe2x80x9cdischarge chargingxe2x80x9d). Some characteristics of the two charging schemes are described with reference to FIG. 1, which shows a relationship between DC applied voltages Vdc from a supply indicated on the abscissa and resultant surface potentials on an electrophotographic photosensitive member on the ordinate.
In the case of discharge charging, as shown in FIG. 1, discharge is initiated only after the voltage applied to the charging member has reached a discharge initiation voltage Vth, and an excess of the applied voltage over the discharge injection provides a surface potential on the photosensitive member. More specifically, in the case of discharge charging using only a DC voltage, a relationship according to the following formula (6) holds between the applied voltage Vdc and the resultant surface potential Vd on the electrophotographic photosensitive member:
|Vd÷|Vdc|xe2x88x92|Vth|xe2x80x83xe2x80x83(6).
In a typical case, Vth may be calculated according to the following formula based on the Paschen""s law:
Vth=(8837.7xc3x97D)1/2+312+6.2xc3x97D,
wherein D=L/K, L is a thickness (xcexcm) of a photosensitive layer, and K is a dielectric constant of the photosensitive layer.
On the other hand, in the case of injection charging, as shown in FIG. 1, a surface potential on an electrophotographic photosensitive member is nearly equal to a voltage applied to the charging member, and the absence of a threshold like the discharge initiation voltage in the case of discharge charging is a characteristic of this charging scheme. In other words, the satisfaction of a relationship according to the following formula (7) at least suggests the possibility of occurrence of injection charging:
|Vdc|xe2x88x92|Vd| less than |Vth|xe2x80x83xe2x80x83(7).
However, this condition alone does not exclude a case where a higher surface potential Vd is given to the photosensitive member due to triboelectrification. Further, based on a premise that the formula (6) represents discharge charging, in a case of the formula (7) where the value of (Vdcxe2x88x92Vd) is close to Vth, some extent of injection charging may occur but discharge charging is believed to be still predominant.
Accordingly, a charging scheme predominantly governed by discharge charging may be represented by the following formula (8):
|Vth/2| less than |Vdc|xe2x88x92|Vd| less than Vthxe2x80x83xe2x80x83(8),
whereas a charging scheme predominantly governed by injection charging may be represented by the following formula (3):
|Vdc|xe2x88x92|Vc|xe2x89xa6|Vth/2|xe2x80x83xe2x80x83(3).
The case of applying a superposition of a DC voltage Vdc (V) and an AC voltage Vac (V) is applied to an electrophotographic photosensitive member from a charging member is considered with reference to FIG. 2. The charging scheme is generally called an AC/DC-superposed scheme. If the peak-to-peak voltage of an AC voltage is denoted by Vpp (V), in the case of discharge charging wherein Vpp is set so as to satisfy the following formula (9), the surface potential provided to an electrophotographic photosensitive member may be represented by formula (10) below:
|Vpp|xe2x89xa72xc3x97|Vth|xe2x80x83xe2x80x83(9)
|Vd|÷|Vdc|xe2x80x83xe2x80x83(10).
Thus, in the case of AC/DC-superposed discharge charging, the voltage Vpp and Vdc applied to a primary charging member are determined so as to stabilize the charging performance.
However, in the case of a lower Vpp as represented by formula (11) below, the surface potential provided to an electrophotographic photosensitive member may be changed to a value as represented by formula (12) below:
|Vpp| less than 2xc3x97|Vth|xe2x80x83xe2x80x83(11)
|Vd|÷|Vpp/2|+|Vdc|xe2x88x92Vth|xe2x80x83xe2x80x83(12).
In other words, if it is assumed that the DC voltage component Vdc (V) of the applied voltage and the discharge initiation voltage Vth (V) are constant, as the peak-to-peak voltage Vpp (V) of the AC voltage is gradually lowered, the surface potential Vd (V) provided to an electrophotographic photosensitive member is correspondingly lowered and when Vpp is 0, Vd becomes the same as in the DC charging scheme and the formula (12) is reduced to the formula (6). Further, if dark attenuation of potential on the photosensitive member is taken into account, formula (13) below may be more accurate than the formula (12):
|Vd|xe2x89xa6|Vpp/2|+|Vdc|xe2x88x92|Vth|xe2x80x83xe2x80x83(13).
On the other hand, in the AC/DC-superposed charging scheme in case where the injection charging mechanism is predominant, the AC voltage plays only a supplementary role and a high Vpp is not used generally. Thus, only a level of Vpp according to the formula (11) is applied. The injection charging is remarkably different from the discharge charging in that in a charging system wherein the injection charging mode is predominant, the surface potential provided to the photosensitive member is still almost identical to the DC component voltage Vdc of the applied voltage from the charging member even at such a low Vpp level. The difference between the two charging schemes is clearly shown in FIG. 2. In other words, in the charging system wherein the injection charging is predominant, in addition to the holding of the formula (3), formula (14) also holds true instead of the formula (13):
|Vd| greater than |Vpp/2|+|Vdc|xe2x88x92|Vth|xe2x80x83xe2x80x83(14).
As is understood from the above discussion, there is a clear difference in principle between the charging system wherein the injection charging is predominant (which may also be called a xe2x80x9cinjection charging-controlled charging system or schemexe2x80x9d) and the discharge charging system regardless of whether they are operated in the pure DC-application mode or the AC/DC-superposed application mode.
In the injection charging-controlled charging scheme, discharge is not substantially caused as charges are directly into the photosensitive member, and accordingly, the occurrence of discharge products, such as NOX and Ozone, and deterioration of the photosensitive member therewith are substantially negligible, and little electrical damage is exerted to the photosensitive member, so that an ideal charging operation can be effected.
However, in order to effectively operate the injection charging scheme, the charging member is caused to contact the photosensitive member with a relative speed difference therebetween, and relatively hard charging particles are retained at a contact region between the charging member and the photosensitive member. Accordingly, in the injection charging-controlled charging system, the photosensitive member surface is liable to receive a large load and be damaged or scarred thereby. Further, an electrophotographic image-forming system including the charging scheme is liable to suffer from the difficulty of fog in continuous image formation in the high humidity environment peculiarly inherent to the charging system.
A principal object of the present invention is to provide an electrophotographic apparatus including an injection charging-controlled charging system, resistant to damages attributable to the charging system and capable of stably providing high-quality images free from fog peculiar to the charging system even after repetitive and continual image formation in a high humidity environment.
Another object of the present invention is to provide a process cartridge suitable for organizing such an electrophotographic apparatus.
According to the present invention, there is provided an electrophotographic apparatus, comprising: an electrophotographic photosensitive member and a charging means,
wherein the charging means comprises a conductor particle-carrying member having an electroconductive and elastic surface, and conductor particles having a particle size of 10 nm-10 xcexcm and carried on the carrying member so as to be disposed in contact with the photosensitive member, thereby directly injecting charges to the photosensitive member to charge the photosensitive member, and
the photosensitive member comprises a photosensitive layer and a charge injection layer as a surface layer disposed in this order on a support, the charge-injection layer having a thickness d (xcexcm) and an elastic deformation percentage We (OCL) (%) satisfying a relationship of formula (1) below with an elastic deformation percentage We (CTL) (%) of the photosensitive layer:
xe2x88x920.71xc3x97d+We(CTL)xe2x89xa6We(OCL)xe2x89xa6
0.03xc3x97d3xe2x88x920.89xc3x97d2+8.43xc3x97d+We(CTL)xe2x80x83xe2x80x83(1).
According to the present invention, there is also provided a process cartridge which includes the above-mentioned electrophotographic photosensitive member and charging means integrally supported to form a unit detachably mountable to an electrophotographic apparatus.